* Senate vote delayed on U.S. aid to Ukraine (Recasts with Kerry quotes to House of Representatives, White House comments, Senate vote delayed)

By Lesley Wroughton

WASHINGTON, March 13 (Reuters) - The United States and the European Union will take serious steps against Russia if a referendum planned for Sunday in Ukraine's Crimea region results in Russian annexation, U.S. Secretary of State John Kerry said on Thursday.

Kerry told a Senate hearing that he hoped to avoid such a response through last-ditch discussions on Friday with his Russian counterpart, Foreign Minister Sergei Lavrov, in London.

At a House of Representatives hearing later, Kerry suggested that the referendum would go ahead and that any sanctions depended on Moscow's response.

"If we are not successful tomorrow in finding a way forward and the referendum - which we all anticipate is going to take place on Sunday - is done without some path forward, there are going to be serious repercussions," Kerry told the House Foreign Relations Committee.

Kerry appeared before the panel to discuss the State Department's budget request for 2015.

The United States has said it recognizes the historic ties between Crimea and Russia but is opposed to the takeover of the Black Sea peninsula by Russian forces. The United States said that action violated Ukraine's sovereignty and international law. The Crimean takeover followed the ouster of a Russian-backed Ukrainian government after months of protests.

Even as tensions mounted over Crimea and Russia launched new military exercises near Ukraine, Kerry cautioned against "hysteria or excessive concern" over the possibility that Russian forces could take over Ukraine.

"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps in Europe and here," Kerry told the Senate Appropriations Committee on Foreign Operations earlier in the day.

The United States has already prepared the way for imposing visa bans and asset freezes on Russian and Ukrainian individuals and companies.

At a regular news briefing, White House spokesman Jay Carney said, "Should Russia continue down the path that it is currently on and move forward with an attempt to annex Crimea or to in other ways continue to violate Ukraine's sovereignty, there will no doubt be additional costs."

While not providing details of what additional sanctions the United States might pursue, Carney said, "I don't think Russia has any doubt that we have the tools available and the authority available to heighten the cost to Russia for the transgressions that have occurred."

AID TO UKRAINE

In the Senate, a vote is unlikely until later this month at the earliest on a bill providing aid to Ukraine, congressional aides said, as lawmakers debate whether International Monetary Fund reforms should be included in the package.

Legislation including the IMF reforms, loan guarantees for Ukraine, potential sanctions against Russians and Ukrainians and economic aid for Ukraine's new government was approved Wednesday by the Senate Foreign Relations Committee.

The measure will not be considered by the full Senate until after Congress returns from a week-long recess beginning Friday, aides said. The measure would also have to pass the Republican-led House, where it faces a tough fight, before President Barack Obama could sign it into law.

House Speaker John Boehner urged the Senate to pass a House version of a bill backing $1 billion in loan guarantees that does not include the other provisions of the Senate bill, particularly the IMF funding.

In the European Union, a framework to match U.S. sanctions has already been agreed on. The EU has 10 times as much trade with Russia as the United States and depends on Russian natural gas.

Moscow has pledged to respond in kind to any Western sanctions, and China's ambassador to Germany warned of a "spiral" of sanctions hurting both sides.

Kerry told the Senate panel the United States believes there are about 20,000 Russian troops in Crimea but those forces are not in a position to move into the rest of Ukraine. Under an agreement, Russia can have up to 25,000 troops at a military base in Crimea, he said.

"We have contingencies, we are talking through various options that may be available, but our hope is not to create hysteria or excessive concern about that at this point," Kerry said.

"Our hope is to be able to avoid that, but there is no telling that we can," he said. "We make the judgment at this point that they don't have the assets in the places necessary to be able to march in and take over all of Ukraine, but that could change very quickly, and we recognize that."

Citing discussions with Ukrainian officials on the possibility of a Russian invasion, Kerry said they believed "there would probably not be an all-out confrontation initially, but you would have a longtime insurgency."

Ukraine has already called up all military reserves and ordered its armed forces to be combat-ready in case Russia deploys troops in Ukraine. (Additional reporting by Doina Chiacu, Patricia Zengerle, Mark Felsenthal and Will Dunham; Editing by Bill Trott and Jonathan Oatis)